1. Field of the Invention
This invention relates to an ultrasonic method and system for detecting passengers at a door of a transit vehicle without the need to define a plurality of dead zones.
2. Description of Related Art
Present ultrasonic detection methods used to detect and respond to the presence of passengers in the vicinity of a bus or other transit vehicle doorway are based on first detecting and memorizing a signature table for a “standard” environment, for example, with the doors closed and no passengers or objects in the vicinity other than permanent equipment. The table consists of echo response times that define the echoes from permanent equipment. These times are represented by a one bit digitalization of the echo signal in a memory or register position corresponding to echo return time. The amplitude of the echo is not considered. See, for example, Kuhn U.S. Pat. No. 5,148,410 entitled “Sonar Detector for Exiting Passengers.” When a passenger or object not previously in the vicinity of the door causes an echo response at a time not previously recorded, the passenger or object is considered detected. If, however, the echo from a passenger or object occurs at the same time (within established tolerances) as previously memorized, the response is ignored. There is no way to discriminate that response from a response due to permanent bus equipment. In effect, the memorized responses with their tolerances represent “dead zones.”
Repeatedly integrating the echoes following shifted multiple echo pluses to discriminate targets that are straight-on from off-axis targets has been suggested in Magori U.S. Pat. No. 4,634,947 entitled “Method for Evaluating Echo Signals of an Ultrasonic Sensor on a Robot Arm.”